1. Field of the Invention
This invention is generally related to apparatus for effecting two-to-four wire conversion in communication systems. More particularly, the invention is directed to those arrangements utilizing electronic hybrid circuitry for interconnecting a two-wire subscriber loop to a four-wire transmission medium while simultaneously providing DC battery feed to said subscriber loop by means of the hybrid arrangement.
2. Description of the Prior Art
In communication systems, it is presently common practice to transmit signals between a subscriber station and the local switching office via a two-wire bidirectional communication path. However, it has sometimes become necessary, especially in the telephone toll network, to use a pair of one-way communication paths for further transmission of such communication signals originating from two-wire paths. Such a pair of one-way paths is commonly referred to as a four-wire communication path. Therefore, as is well-known in the telephony field, it has become necessary to provide a so-called hybrid arrangement to properly direct signal transmission between the two-wire bidirectional communication path and the input and output portions of the four-wire communication path, while preventing the coupling of four-wire input signals to the output portion of the four-wire path. Such preventative isolation is known in the art as the transhybrid balance function.
The classic approach to implementing the required hybrid functions involved the use of transformer coupling. However, the transformer approach has inherent disadvantages including excessive physical size and weight, electromagnetic cross-coupling among adjacently located hybrid units, and deleterious effect on return and transhybrid loss caused by basic transformer electrical operating characteristics.
Prior art approaches utilizing electronic hybrid circuits have been proposed to overcome the inherent difficulties associated with transformer arrangements. However, even these prior electronic designs present problems of inefficient operation and lack of transhybrid balance adjustment capability as a function of two-wire path complex impedance variation. Additionally, the prior electronic approaches lack the capability for overcoming temporary longitudinal imbalance in the two-wire communication path.
With the recent introduction of digital switching technology to local switching systems, an increasing need has arisen for a hybrid arrangement at the interface of two-wire subscriber lines with the switching network. One type digital switching arrangement employing time division multiplexing of pulse-code-modulated digital voice signals requires an interface at the two-wire subscriber line for coupling to suitable coder/decoder apparatus -- i.e., analog voice signals originating from the subscriber station set must be coupled to the digital encoder input, while analog signals from the decoder output must be coupled for transmission towards the station set and must simultaneously be prevented from reaching the encoder input.
The use of a subscriber line hybrid, as in the foregoing digital switching system example, presents an additional hybrid arrangement functional requirement not successfully addressed by the prior art, namely the provision at the hybrid of DC battery feed potential to the subscriber's two-wire communication path. Prior art approaches require the use of special battery feed devices at the two-wire path (e.g., inductive feed coils) to allow for efficient amplifier operation within the electronic hybrid. However, the use of such device once again introduces the adverse effects of increased size, weight, and noise coupling among adjacent hybrid circuits.
Hence, there is seen to be a need in the hybrid field for a truly inductorless, transformerless approach to implementing all of the classical hybrid functions in addition to meeting a further requirement for efficiently providing DC operating power to a two-wire subscriber line.